Steam turbine and method of measuring the vibration of a moving blade in a flow passage of a steam turbine

ABSTRACT

The invention relates to a steam turbine ( 3 ) comprising an optical measuring system ( 60 ) for measuring a moving blade vibration. A transmitter ( 55 ) produces a light beam ( 61 ) which strikes the moving blades ( 27, 29, 31, 33 ) and is reflected by these into a receiver ( 57 ). By providing that the transmitter ( 55 ) is separated from the receiver ( 56 ), the invention achieves a measuring angle that actually reduces the scattered light effect of the steam enough to enable reliable optical measurements of the blades vibration. The invention also relates to a method for measuring the vibration of a moving blade ( 33 ) in a flow channel ( 19, 21, 23, 25 ) of a steam turbine ( 3 ).

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP01/10599 which has an Internationalfiling date of Sep. 13, 2001, which designated the United States ofAmerica and which claimed priority on European application No. EP00120036.9 filed Sep. 14, 2000, the entire contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The invention generally relates to a steam turbine having a flow passagein which a moving blade is arranged. More preferably, it relates to oneincluding a measuring system for measuring a vibration of the movingblade. The invention also generally relates to a method of measuring thevibration of a moving blade in a flow passage of a steam turbine.

BACKGROUND OF THE INVENTION

The technical field of the invention is the measurement of bladevibrations in fluid-flow machines. Moving blades in fluid-flow machinesare subjected to high loads. A vibration may be induced in them onaccount of alternating stresses, this vibration, if it lies in thevicinity of a natural vibration in the respective blade, leading toespecially high mechanical stresses of this blade. In order to detectsuch especially high loads in good time, the vibration states of theblades are measured in different ways.

U.S. Pat. No. 4,996,880 discloses a steam turbine and a method ofmeasuring the vibration of a moving blade in the flow passage of a steamturbine. Here, the vibration of the moving blade is measured by anacoustic signal. A Doppler displacement which is caused by the movementof the moving blade is measured with an acoustic sensor. Depending onthe vibration state of the moving blade, a characteristic Doppler signalis obtained, so that, in inverse relationship to the Doppler signal, thevibration state of the moving blade can be deduced.

U.S. Pat. No. 4,518,917 shows a method of measuring the vibration stateof moving blades, in which method the distance of the blades from thesurrounding casing is measured. An impedance dependence of sensorsarranged in the casing is utilized in this case. Depending on thedistance of a moving blade from the sensor, an impedance change isobtained.

A further method of measuring the vibration of a moving blade has beendisclosed by U.S. Pat. No. 4,934,192. Here, an axial deflection onaccount of an axial vibration of the moving blade is measured by twosensors being arranged symmetrically over a prominence on the tip of themoving blade in the rest state of the latter. The sensors are eachdesigned as an electrical winding in which a voltage is induceddepending on the distance from the prominence on the tip of the movingblade. In the event of an asymmetrical arrangement of the sensorsrelative to the tip of the moving blade, this asymmetrical arrangementoccurring on account of an axial vibration of the moving blade, adifferential signal is produced which characterizes the blade vibration.

In the paper “A Review of Analysis Techniques for Blade Tip TimingMeasurements”, S. Heath, M. Imregum, ASME publication 97/GT/218,presented at the International Gas Turbine and Aeroengine Congress andExhibition, Orlando, Fla., USA, May 2, 1997, a method of measuring theblade vibration by means of a laser is described. However, this methodrelates solely to gas turbines.

The methods for the non-contact measurement of the vibration of a movingblade are either comparatively inaccurate or require a magnetizablematerial for the moving blade or the awkward and unreliable placement ofa magnetic marking on the moving blade.

SUMMARY OF THE INVENTION

Accordingly, an object of an embodiment of the invention is to specify asteam turbine in which the measurement of the vibration of a movingblade is possible in a non-contact and reliable manner largelyindependent of properties of the moving blade. A further object of anembodiment of the invention is to specify a corresponding method ofmeasuring the vibration of a moving blade.

According to an embodiment of the invention, an object which relates toa steam turbine may be achieved by a steam turbine having a flow passagein which a moving blade is arranged, and having a measuring system formeasuring a vibration of a moving blade. The measuring system mayinclude a transmitter for emitting a light beam to the moving blade anda receiver for receiving the light beam reflected from the moving blade,and the transmitter being separate from the receiver.

The use of an optical measuring system for measuring the vibration stateof a moving blade in a flow passage of a steam turbine has hitherto noteven been taken into consideration. This is due to the fact that,according to the prevailing opinion, the steam flowing in the flowpassage makes optical detection of the moving blade virtually impossibleon account of high scatter in the steam. In this case, however,measuring systems have hitherto always been based on a transmitter andreceiver combined in a unit, so that in principle measurements are takenin backscatter. Such systems are used, for example, for measuring bladevibrations of gas turbines. They offer the advantage that only thecasing part defining the flow passage need be interfered with forfitting the transmitter and receiver.

According to the findings of an embodiment of the invention, such anoptical measuring system may now also be used in a steam turbine formeasuring a blade vibration if the rigid concept of the coupledtransmitter/receiver unit is dispensed with. This is because, in asuitable arrangement of transmitter and receiver, the proportion ofscattered radiation which is caused by the steam can now be kept so lowthat a reliable measurement of blade vibration is made possible.

The transmitter and receiver need not be designed as a completelylight-producing or light-converting unit; they may also be designed, forexample, as a glass fiber cable and direct a light beam from a lightsource or to a converting unit, such as a photocell for instance.

The transmitter is preferably designed for emitting a laser beam. Onaccount of its monochromasy and low divergence, a laser beam isespecially suitable for the measurement.

The transmitter and receiver are preferably arranged in such a way thatthe transmitted light beam and the reflected light beam enclose an angleof reflection of at least 45° with one another. The angle of reflectionis also preferably greater than 90°. In such a large-angled arrangement,a good ratio of light beam reflected directly into the receiver toscattered radiation caused in the steam is obtained, since theproportion of scattered radiation drops with a larger angle.

The transmitter is preferably set in such a way that the transmittedlight beam illuminates an area of less than 1 mm² on the moving blade.High focusing or a low divergence of the light beam likewise encouragesa low proportion of scattered radiation.

The receiver, in a casing defining the flow passage, is preferablyarranged so as to be set back from the flow passage in such a way that,apart from the directly reflected light beam, at most a small proportionof scattered radiation reaches the receiver. By the receiver being setback in the casing, a diaphragm, as it were, is constructed, and thisdiaphragm essentially allows only such light to reach the receiver whichspreads in a virtually rectilinear direction from the illuminated areaon the moving blade to the receiver. As a result, scattered radiationspreading at other angles is mostly screened off before entering thereceiver.

The moving blade is preferably made of a non-magnetic material. Themoving blade is also preferably made of a titanium-based alloy. Here,the expression “non-magnetic” means that the material of the movingblade has no appreciable ferromagnetic properties. Especially in thecase of such a material, a simple non-contact measurement by means ofmagnetic induction is ruled out. Such a material is, for example, atitanium-based alloy, which are used in new generations of steamturbines, in particular when the moving blades of the last stages oflow-pressure parts are very large. However, such large blades especiallyare susceptible to vibration excitation and are loaded to a considerableextent by vibrations. Here, especially, a reliable monitoring system formeasuring the blade vibration states must therefore be used. By means ofthe optical monitoring of the blade vibrations, this is also possiblefor such blades in a simple and reliable manner.

According to an embodiment of the invention, an object which relates toa method may be achieved by a method of measuring the vibration of amoving blade in a flow passage of a steam turbine, a light beam beingdirected onto the moving blade and being reflected from the latter at anangle of reflection greater than 45° and directed to a receiver, and theblade vibration being calculated from the signal thus received.

The advantages of such a method follow in accordance with the abovestatements in relation to the advantages of the steam turbine.

The angle of reflection is preferably greater than 90°. The light beamused is preferably a laser beam.

The light beam is preferably directed onto a reflecting surface, lyingon the moving blade, in such a way that the illuminated part of thereflecting surface is less than 50 mm².

The moving blade is preferably made of a non-magnetic material, alsopreferably of a titanium-based alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail by way of example withreference to the drawings. In the drawings, partly schematically and notto scale:

FIG. 1 shows a steam turbine, and

FIG. 2 shows a measuring system for measuring the vibration of a movingblade in a steam turbine.

The same reference numerals have the same meaning in the differentfigures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a steam turbine 3. A high-pressure part 9, anintermediate-pressure part 11 and a double-flow low-pressure part 13 arearranged one behind the other on a common shaft 5 between bearings 7.The double-flow low-pressure part 13 consists of a first low-pressurehalf 15 and a second low-pressure half 17. The low-pressure part 13 hasa low-pressure casing 14. The intermediate-pressure part 11 has anintermediate-pressure casing 12. The high-pressure part 9 has ahigh-pressure casing 10. The high-pressure part 9 has a high-pressureflow passage 19. The intermediate-pressure part 11 has anintermediate-pressure flow passage 21. The low-pressure part 13 has afirst low-pressure flow passage 23 in the first low-pressure half 15 anda second low-pressure flow passage 25 in the second low-pressure half17. Moving blades 27 are arranged in successive moving-blade rings inthe high-pressure flow passage 19. Intermediate-pressure moving blades29 are arranged in successive moving-blade rings in theintermediate-pressure flow passage 21. Moving blades 31 are arranged insuccessive moving-blade rings in the low-pressure flow passage 23 of thefirst low-pressure half 15. Moving blades 33 are arranged in successivemoving-blade rings in the low-pressure flow passage 25 of the secondlow-pressure half 17 of the low-pressure part 13.

During operation of the steam turbine 3, live steam is fed to thehigh-pressure part 9 via a high-pressure steam feed line 41, this livesteam partly expanding in the high-pressure part 9 and converting energyinto energy of rotation of the shaft 5 in the process. The partlyexpanded steam is then directed via an intermediate-pressure feed line43 to the intermediate-pressure part 11, where it expands further andtransmits further energy of rotation to the shaft 5. The steam is thendirected in double flow via a low-pressure steam feed line 45 to thelow-pressure part 13 in such a way that it flows in parallel through thefirst low-pressure half 15 on the one hand and through the secondlow-pressure half 17 on the other hand. In the process, the steamexpands further and further energy of rotation is transmitted to theshaft 5. The largely expanded steam is then fed via discharge lines 47to a condenser (not shown). Also not shown for the sake of clarity areguide blades, which are arranged alternately to the moving blades in theaxial direction in the flow passages.

The steam flowing past the moving blades may lead to vibrations beinginduced in the moving blades. This applies in particular to the verylarge moving blades 31, 33 of the low-pressure part 13, to be precise inparticular to the last stages of the low-pressure part 13. Suchvibrations may result in the moving blades 31, 33 being loaded in such away that the service life is reduced. In order to detect this in goodtime, the vibration state of each moving blade 31, 33 is monitored. Withprevious measuring techniques, this was only possible in a complicatedmanner, or virtually not all in the case of non-magnetic moving blades31, 33.

FIG. 2 shows a detail of the second low-pressure half 17 of FIG. 1. Ameasuring system 60 is arranged in the low-pressure casing 14. Themeasuring system 60 includes a transmitter 55 and a receiver 57. A lightbeam 61, formed by a laser beam, is directed by the transmitter 55 intothe flow passage 25 and onto a moving blade 33. The moving blade 33 hasa shroud band 51 which has a bevelled reflecting surface 53 toward thetransmitter 55. From the reflecting surface 53, the light beam 61 isdirected as reflected light beam 63 to the receiver 57. The transmitter55 and the receiver 57 are separated from one another in such a way thatan angle x greater than 90° is obtained between the incident light beam61 and the reflected light beam 63. On the reflecting surface 53, thelight beam 61 illuminates a part area 65 which is less than 1 mm².

Due to the separate arrangement of transmitter 55 and receiver 57, it ispossible to take a reflection measurement even in the flow passage 25,through which steam flows, of the steam turbine 3. Whereas according tothe prevailing opinion the steam should result in far too much scatteredlight being produced, as a result of which an optical measurement wouldnot be realizable, the arrangement shown, on account of thecomparatively large angle x, offers the possibility of a reliableoptical measurement without interference by scattered light. This isfurther assisted by a low divergence of the light beam 61 and by thereceiver 57 being set back in the low-pressure casing 14, as a result ofwhich a channel-like diaphragm is produced which largely screens offscattered light.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A steam turbine, comprising: a flow passage inwhich a moving blade is arranged; and a measuring system for measuring avibration of the moving blade, the measuring system including, atransmitter for emitting a light beam toward the moving blade, and areceiver for receiving the light beam reflected from the moving blade,the transmitter being separate from the receiver, wherein thetransmitter and receiver are arranged in at least one casing part of thesteam turbine and wherein the receiver, in a casing defining the flowpassage, is arranged so as to be set back from the flow passage in sucha way that, apart from the directly reflected light beam, at most asmall proportion of scattered radiation reaches the receiver.
 2. Thesteam turbine as claimed in claim 1, wherein the transmitter is designedfor emitting a laser beam.
 3. The steam turbine as claimed in claim 1,wherein the transmitter and receiver are arranged such that thetransmitted light beam and the reflected light beam enclose an angle ofreflection of at least 45° with one another.
 4. The steam turbine asclaimed in claim 3, wherein the angle of reflection is at least 90°. 5.The steam turbine as claimed in claim 1, wherein the transmitter is setsuch that the transmitted light beam illuminates an area of less than 1mm² on the moving blade.
 6. The steam turbine as claimed in claim 1,wherein the moving blade is made of a non-magnetic material.
 7. Thesteam turbine as claimed in claim 6, wherein the moving blade is made ofa titanium-based alloy.
 8. A method of measuring a vibration of a movingblade in a flow passage of a steam turbine, comprising: directing alight beam onto a moving blade, the light beam being reflected from themoving blade at an angle of reflection greater than 45°; directing thereflected light beam to a receiver; and calculating the blade vibrationfrom the received reflected light beam, wherein the light beam isdirected onto a reflecting surface on the moving blade, in such a waythat the illuminated part of the reflecting surface is less than 50 mm².9. The method as claimed in claim 8, wherein the angle of reflection isgreater than 90°.
 10. The method as claimed in claim 8, wherein thelight beam is a laser light beam.
 11. The method as claimed in claim 8,wherein the moving blade is made of a non-magnetic material.
 12. Themethod as claimed in claim 8, wherein the moving blade is made of atitanium-based alloy.
 13. A measuring system for measuring a vibrationof a moving blade, arranged in a flow passage of a steam turbine,comprising: a transmitter for emitting a light beam toward the movingblade, and a receiver for receiving the light beam reflected from themoving blade, the transmitter being separate from the receiver, whereinthe transmitter and receiver are arranged in at least one casing part ofthe steam turbine and wherein the receiver, in a casing defining theflow passage, is arranged so as to be set back from the flow passage insuch a way that, apart from the directly reflected light beam, at most asmall proportion of scattered radiation reaches the receiver.
 14. Themeasuring system as claimed in claim 13, wherein the transmitter isdesigned for emitting a laser beam.
 15. The measuring system as claimedin claim 13, wherein the transmitter and receiver are arranged such thatthe transmitted light beam and the reflected light beam enclose an angleof reflection of at least 45° with one another.
 16. The measuring systemas claimed in claim 15, wherein the angle of reflection is at least 90°.17. The measuring system as claimed in claim 13, wherein the transmitteris set such that the transmitted light beam illuminates an area of lessthan 1 mm² on the moving blade.
 18. The measuring system as claimed inclaim 13, wherein the moving blade is made of a non-magnetic material.19. The measuring system as claimed in claim 13, wherein the movingblade is made of a titanium-based alloy.